Electronic phase separation in the rare-earth manganates (La_1−xLn_x)_0.7Ca_0.3MnO₃ (Ln=Nd, Gd and Y)

Abstract

Electron transport and magnetic properties of three series of manganates of the formula (La_1−xLn_x)_0.7Ca_0.3MnO₃ with Ln=Nd, Gd and Y, wherein only the average A-site cation radius <r_A> and associated disorder vary, without affecting the Mn⁴⁺/Mn³⁺ ratio, have been investigated in an effort to understand the nature of phase separation. All three series of manganates show saturation magnetization characteristic of ferromagnetism, with the ferromagnetic T_c decreasing with increasing x up to a critical value of x, x_c (x_c=0.6, 0.3, 0.2 respectively for Nd, Gd, Y). For x>x_c, the magnetic moments are considerably smaller, showing a small increase around T_M, the value of T_M decreasing slightly with increase in x or decrease in <r_A>. The ferromagnetic compositions (x≤x_c) show insulator-metal transitions, while the compositions with x>x_c are insulating. The magnetic and electrical resistivity behaviour of these manganates is consistent with the occurrence of phase separation in the compositions around x_c, corresponding to a critical average radius of the A-site cation, <r_A^c>, of 1.18 Å. Both T_c and T_IM increase linearly when <r_A>><r_A^c> or x≤x_c, as expected of a homogeneous ferromagnetic phase. Both T_c and T_M decrease linearly with the A-site cation size disorder as measured by the variance σ². Thus, an increase in σ² favours the insulating AFM state. Percolative conduction is observed in the compositions with <r_A>><r_A^c>. Electron transport properties in the insulating regime for x>x_c conform to the variable-range hopping mechanism. More interestingly, when x>x_c, the real part of dielectric constant (ε') reaches a high value (10⁴-10⁶) at ordinary temperatures dropping to a very small (~500) value below a certain temperature, the value of which decreases with decreasing frequency.